The present invention is in the field of polarization sensitive beam splitting techniques and relates to a beam polarizer device based on the effect of double refraction of light.
Beam polarizers are well known optical devices that are widely used as filters for allowing the passage of light polarized in one direction only, or for image separation. Beam polarizer devices of the kind specified above, employing the effect of double refraction of light, are used in applications which need two spatially separated light components of the different polarizations to be produced from an unpolarized light beam. Such a device typically comprises two spaced-apart glass wedges and a polarization sensitive media therebetween. The term xe2x80x9cdifferent polarizationsxe2x80x9d signifies different orientations of the electric fields of a light wave, which are mutually perpendicular, each being perpendicular to the direction of propagation of an unpolarized beam impinging onto a polarization sensitive media.
The polarization sensitive media is typically in the form of either a plane-parallel, solid-state, birefringent plate, or a multi-layered dielectric structure. The production of the plate-like birefringent media requires the use of natural crystals of high optical quality such as, for example, calcite. Unfortunately, the natural crystals of large sizes cannot be easily obtained. The use of a multi-layered structure provides high polarization purity in one of the channels, unless a very complicated structure up to forty layers is employed. However, even employing such a complicated multi-layered structure, the beam polarizer suffers from a drawback consisting in an unavoidable requirement of a substantially small xe2x80x98acceptance anglexe2x80x99, i.e. the angle of incidence of a beam of radiation to be split onto a beam polarizer device. This is caused by the fact that the operation of the multi-layered structure (which is typically formed of different dielectric materials) is based on an interference phenomenon which allows for an acceptance angle not exceeding 3xc2x0.
Beam polarizer devices employing a liquid crystal (LC) cell as a birefringent medium have been developed and disclosed, for example, in the article xe2x80x9cUsing the Interface Between Glass and a Nematic Liquid Crystal for Optical-Radiation Polarization Over a Broad Spectral Rangexe2x80x9d, A. A. Karetnikov, Opt. Spectrosk. (USSR), 67, 324-326, August 1989. Such a device is schematically illustrated in FIG. 1 being generally designated 1. The device 1 comprises a conventional LC cell 2 located between parallel sides 4a and 6a of a pair of glass prisms 4 and 6. The LC cell 2 typically comprises a layer 8 formed of a nematic liquid crystal material (NLC), which is enclosed between two so-called xe2x80x98orienting layersxe2x80x99 10a and 10b formed on the sides 4a and 6a. The orienting layers 10a and 10b, which are in the form of thin polymer films, provide a homogeneous orientation of the long axes, generally at AX, of rod-like molecules 12 of the NLC, defining thereby the orientation of an optical axis of the layer 8. The molecules 12 are oriented at a certain so-called xe2x80x9cpre-tilt anglexe2x80x9d xcexa8(0xc2x0 less than xcexa8 less than 90xc2x0) relative to the surface 2a of the LC cell 2.
The device 1 operates in the following manner. An unpolarized light wave 14 impinges from the glass 4 onto the surface 2a at an angle xcfx86. The surface 2a of the LC cell represents an interface on which two different light components contained in the unpolarized wave 14 are spatially separated into so-called xe2x80x9cordinaryxe2x80x9d and xe2x80x9cextraordinaryxe2x80x9d beams 16 and 18, respectively.
The terms xe2x80x9cordinary beamxe2x80x9d and xe2x80x9cextraordinary beamxe2x80x9d used herewith signify the beams of different polarizations produced by the passage of an unpolarized light beam through a crystal. The xe2x80x9cordinary beamxe2x80x9d is that which obeys Snell""s Law and gives a constant refraction index for all angles of incidence, while the xe2x80x9cextraordinary beamxe2x80x9d is that which does not obey Snell s Law. The different polarizations are defined by different orientations of the electric fields of a light wave relative to a plane of polarization. The plane of polarization, generally designated 20, is such a plane that contains beams impinging onto and reflected from the birefringent cell, i.e. beams 14 and 16, and a normal ON to the cell""s surface.
Thus, the beam 16 contains a light component having one of the two orientations of the electric field, i.e. one polarization, while the beam 18 contains a light component of the other orientation of the electric field, i.e. the other polarization. As indicated above, these different orientations of the electric field are mutually perpendicular, each being perpendicular to the direction of propagation of the beam 14 impinging onto the LC cell""s surface 2a. 
The beam 18 propagates inside the prism 6 defining an angle xcex8 between the direction of its propagation and the optical axis AX of the NLC layer 8. The NLC layer 8, similar to uniaxial crystal plate, is characterized by refraction indices nor and nex for ordinary and extraordinary beams 16 and 18, respectively. It is known that the refraction index nex represents a function of the angle xcex8 and is associated with the refraction index nor, as follows:
nex(xcex8)=Nprxc2x7Nex(nor2 sin2 xcex8+Nex2 cos2 xcex8)xe2x88x92xc2xd
wherein Npr is the refraction index of the glass prisms; Nex is the fundamental value of the refraction index for an extraordinary beam, that is:
Nex=nex(90xc2x0)
The refraction index Npr is chosen to be as follows:
Npr≈Nex
Npr≈nex(xcex8)
If the angle of incidence xcfx86 satisfies the following condition:
xcfx86 greater than xcfx86cr
wherein xcfx86cr is a critical angle defined by Snell""s Law, then the ordinary beam 16 undergoes a total internal reflection (TIR), while the extraordinary beam 18 propagates inside the LC cell 2 with a divergence angle xcfx86. The angle of orientation xcex8 of the direction of propagation of the beam 18 relative to the optical axis AX is associated with the divergence angle xcfx86xe2x80x2 and with the angle of orientation "psgr" of the optical axes AX relative to the surface 2a as follows:
xcex8=90xc2x0+xcfx86xe2x80x2xe2x88x92"psgr"
It is thus understood that the existence of the pre-tilt angle "psgr" significantly influences the above conditions related to the refraction index nex. Obviously, if a beam polarizer is a so-called xe2x80x9cactive devicexe2x80x9d, a certain desired value of a pre-tilt angle can be obtained by means of an electric field appropriately applied across the LC layer. However, the case may be such that a beam polarizer is a so-called xe2x80x9cpassive devicexe2x80x9d and, therefore, such an application of the electric field is either undesirable or ineffective.
It is often the case that a beam polarizer device is the constructional part of a complicated polarization sensitive optical system such as, for example, projection display. This requires the maximum purity of two different polarizations, the preset orientations thereof relative to the plane of polarization and a substantially wide range of the acceptance angle.
It is an object of the present invention to provide a novel beam polarizer device and a method for its manufacturing for splitting an unpolarized radiation into a pair of spatially separated radiation components of different polarizations.
It is a further object of the present invention to provide such a beam polarizer device in which the different polarizations are substantially fully separated from each other.
There is thus provided, according to the present invention, a beam polarizer device for splitting an unpolarized beam of incident radiation into first and second beams of different polarizations, said beam polarizer comprising:
a birefringent cell interposed between a pair of parallel sides of first and second prisms made of an optically transparent material;
wherein the birefringent cell is formed of an oriented organic material having a desired orientation of its optical axis relative to said sides of the prisms;
wherein the organic material has substantially different refraction indices n1 and n2 for light components of, respectively, two different orientations of electric fields relative to the direction of propagation of a beam impinging onto the birefringent cell while propagating inside the first prism;
wherein said optically transparent material has a refraction index n3 which is substantially equal to the greatest one between the refraction indices n1 and n2.
Thus, the idea of the present invention is based on the following main features. The organic material is formed of elongated molecules having their long axes. The term xe2x80x9coriented organic materialxe2x80x9d used herewith, signifies that the elongated molecules have a homogeneous orientation of the long axes, defining thereby an orientation of the optical axis of the birefringent cell.
The organic material may be in the form of a stretched polymer film such as, for example, polycarbonate or mylar. To this end, the film is stretched in a manner to provide the desired orientation of its optical axis. Preferably, the optical axis of the film is either perpendicular or parallel to the sides of the prism enclosing the film therebetween.
Alternatively, the organic material may be a liquid crystal (LC). In this case, the birefringent cell is in the form of a conventional LC cell comprising an LC layer interposed between a pair of oriented layers formed on the parallel sides of the first and second prisms enclosing the LC cell. The optical axis of the LC cell is oriented at a desired angle "psgr" relative to the parallel sides of the prisms enclosing the LC cell. To this end, both the LC and orienting materials are selected so as to provide the desired value of the angle "psgr". Additionally, the orienting layer may be specifically processed so as to adjust the value of the angle "psgr". Preferably, each of said orienting layers has a thickness less than xcex/4 wherein xcex is the shortest wavelength in a wavelength range of the incident radiation.
The first and second prisms may be symmetrically identical. Each of the prisms may be in the form of a Dove prism.
Preferably, the device also comprises an additional birefringent cell formed on an outer surface of that side of the first prism which is parallel to the sides enclosing the birefringent cell. The additional birefringent cell is constructed similar to the birefringent cell enclosed between the parallel sides of the first and second prisms. More specifically, the additional birefringent cell is formed of organic and optically transparent materials, having the similar relationship between their refraction indices as the birefringent cell enclosed between the first and second prisms. An absorbing plate may cover the additional cell at its outer side.
Preferably, the device also comprises a pair of diffraction gratings of a predetermined design, located at opposite sides of the birefringent cell.
Preferably, at least one of those ribs of the device which define the parallel sides of the first and second prisms is in the form of an elongated pit having beveling edges. This at least one pit is filled with gluing and sealing materials.
According to another aspect of the present invention, there is provided a method for manufacturing a beam polarizer device for splitting an unpolarized beam of incident radiation into first and second beams of different polarizations, said beam polarizer comprising a birefringent cell interposed between a pair of parallel sides of first and second prisms made of an optically transparent material, the method comprising the steps of:
(a) selecting an organic birefringent material having elongated molecules and substantially different refraction indices n1 and n2 for light components of, respectively, two different orientations of electric fields contained in a light wave relative to the direction of its propagation while impinging onto the birefringent cell;
(b) orienting the elongated molecules of the selected organic material so as to provide a desired orientation of its optical axis relative to said sides of the prisms;
(c) selecting the optically transparent material having a refraction index n3 which is substantially equal to the greatest one between the refraction indices n1 and n2.
Thus, the beam polarizer device constructed according to the invention, namely by selecting the materials of the device so as to provide the desired relationship between their refraction indices and by obtaining a desired orientation of the optical axis of a birefringent cell, is capable of providing substantially pure separation of the different polarizations within a substantially wide range of an acceptance angle(about xc2x110xc2x0).